There has been a long felt desire for a modulated pressure relief valve on a vessel for venting fluid when the pressure exceeds a desired dynamic set-point or threshold. A particular desire is for a modulated pressure relief valve with a threshold that could vary according to process requirements and its upper limit could serve to avoid blowing a safety relief device such as an expendable rupture disc or pop-off safety relief valve both of which are typically designed for infrequent use. It is desirable to communicate the desired pressure set-point to the relief valve via a reference pressure signal (typically air) that is equal to the desired vessel pressure.
Furthermore, it is desirable to have a modulating pressure relief valve which is very simple, with cleanable parts and with no narrow passageways that can become plugged with debris or frozen product. Such a device would be useful in many applications including the food, beverage, gelatin, and polymer industries, where the product can become frozen in critical passages, and where all crevices must be exposed to the rinsing and cleaning processes.
Typical safety relief valves, such as those exemplified in U.S. Pat. No. 6,095,183, include a valve member biased by a spring into engagement with a seat. If product pressure exceeds a predetermined level, the force against the valve exceeds the biasing force of the spring causing the valve to lift from the seat to vent product pressure.
Typical diaphragm relief valves, such as those exemplified in U.S. Pat. No. 5,944,050, do present simple, cleanable surfaces yet they clearly do not meet the dynamic set-point requirement desired in the art. Furthermore, the inaccuracies of the spring compression typically result in wide variations in relieving pressures, often greater than 10%, and the variations change over time.
Dynamically modulated relief valves are provided that use a reference or pilot signal as exemplified in U.S. Pat. No. 6,318,406. These typically involve complex spring and seal mechanisms. Besides the inherent robustness issues with the complex mechanisms, they are clearly not acceptable for use in processes with debris, freezeable product, or requiring cleanability.
Typical relief valves are quite insensitive to slight differences in pressure, and typically have large cracking pressure biases. A highly sensitive relief valve would help maintain a constant vessel pressure during changes in the upstream process environment which is a feature that is very useful in many industrial steady-state applications, including constant-flow applications.
Further, there has been a long felt desire for a relief valve that could balance vessel pressure exactly to a reference, or pilot, pressure. This would facilitate a complete vessel pressure control system when used in conjunction with a simple instrument pressure sender and a, preferably no-loss, check valve.
Typically available pilot actuated relief valves cannot control vessel pressure to the exact pressure of the reference signal.